I will have the Spicebox, kettle, canopies and tables. I will also have an 100 'extension cord, lights, a hose splitter and hose.

At a bare minimum I can fit 16 butts (4 shelves x 4 butts each)
I might be able to fit 5-6 butts depending on how big the butts are, which would take us to 20-24 butts. I have a fifth shelf which could fit another 4-6 butts but the height clearance on the top rack is a bit lower.